The origin of birds is a very interesting question in evolution.
Over the last few years it has become increasingly certain that birds
evolved from a group of small dinosaurs called dromaeosaurs, part of the bipedal
group known as theropods. Several important
finds, many from China, have been made in the last few years (1) (2) (3)
(4) (5) (6) (7) (8) which support this hypothesis. Fossils have been found
which demonstrate the evolution of feathers and the anatomical structures necessary
for flight. Feathers appear to have evolved well before flight and there is
a considerable controversy around how flight originally evolved:

The two basic hypotheses are the bottom up and the top down
approaches. In the first case, the evolution of flight is seen as a running,
flapping process. In the second, the proto-birds are seen climbing trees
and gliding from tree to tree or to the ground much like flying squirrels do
today. Both hypotheses have some evidence to support them.

Now Xu et al (9) describe a fossil of a dromaeosaur that has
four wings including fully formed feathers on each of its four limbs. The fossil
dates to 124 million to 128 million years. It has been named Microraptor
gui.

The aerodynamic function of the feathers is extremely likely
as they are asymmetric and the asymmetry increases towards the ends of the wings.
M gui has a long tail with feathers - the presence of post caudal vertebrae
is a transitional feature seen only in transitional birds. Xu et al argue that
flight began with arboreal (tree-climbing and dwelling) dromaeosaurs that developed
the ability to glide with four wings. As they further evolved powered
flight, the aerodynamic functionality of the rear wings was lost and birds relied
only on their two (fore-) wings for flying.

The feathers on the tail of M gui (after ref (9))

Further study of the fossil is likely to reveal whether the
anatomy of the skeleton confirms the hypothesis that Microraptor relied on gliding
or whether it was actually capable of powered flight.

The detail associated with understanding the evolution of birds
is increasing with every new transitional fossil found.